Boy, that last post was kind of a downer, wasn’t it? Before I drive all the traffic away with depressing posts, maybe I can get back to hifi and get people thinking again.

Glad I stirred things up with the filament-power post - it even chased an old friend out of the woodwork. (Hey Phil! Give me a shout! I live in Colorado now!)

What I didn’t mention were some of subtler issues of filament power supplies - most importantly, the difference between differential and common-mode noise rejection, which are treated very differently by a filament-powered vacuum tube.

Let’s review how a direct-heated triode works: it amplifies the voltage difference between the grid and virtual cathode. The triode does NOT care about ground or the plate voltage, or any other part of the circuit. The virtual cathode is the electrical center of the filament, which is pretty well balanced - it has 50dB or more rejection of the 2.5, 4, or 5VAC that powers the filament.

This has implications for the filament power supply - noise that rides in differential form is strongly rejected, while anything that shakes BOTH wires has NO rejection at all. Think about it: anything that moves both sides of the filament is amplified just as much as a grid signal!

Even for a simple AC supply, there are implications: noise from the raw AC line will ride right in - moving both sides of the filament - unless special steps are taken to electrostatically screen the filament transformer. If the filament is powered with a winding from the main power transformer, stray capacitance will couple rectification noise pulses from the B+ circuit into the much more sensitive low-voltage circuit. The worst-case scenario employs generic solid-state rectifiers for the B+ rectification and NO electrostatic screening for the AC filament supply - unfortunately, this is exactly how many commercial high-end triode amplifier are built!

The quick-n-dirty “solution” to the inevitable hash and high noise floor of the previous circuit is a DC supply for the filament, typically done with a 10V or so AC supply, a solid-state rectifier bridge, a 1000uF “smoothing” cap, and a 3-pin integrated-circuit voltage regulator. Unfortunately, although this measures fairly well, the measurement measures the wrong thing: it doesn’t capture the noise that appears on BOTH sides of the filament, which the noise that matters to the triode.

All the 3-pin regulator does is reduce differential noise, which is already strongly rejected by the triode. It has NO effect on the common-mode noise, the one that matters, so any noise from the rectifier circuit - or AC line noise that is capacitively coupled through the transformer - sails right into the triode, despite the superficially good measurements. Remember, the entire filament supply is floating up around 50 to 80 volts, and the loop area of the supply acts as an antenna for noise pickup from rectifiers elsewhere in the amplifier. A badly laid-out amplifier could have a loop area of several feet, if the DC supply is on one side and the triodes are on the other side. (The loop area is everything between the triode socket and the AC-powerline side of the supply.)

A better, and more comprehensive solution are chokes in series with the filament - these are desirable for both AC and DC supplies, to improve rejection of high-frequency noise that appears on both sides of the filament. Yes, this means two chokes, or a special common-mode choke, and of course the DC resistance must be a fraction of an ohm, otherwise the filament power is wasted heating the choke. Fortunately, impedances are so low the choke can be in the millihenry range (think subwoofer inductor), but it’s still not going to be compact or light.

To be continued …